Multi-channel audio signals such as those for 5.1ch or 7.1ch have become widely used not only for movies and music but also for games. The reproduction of audio signals using multi-channel speakers placed in predetermined positions surrounding a listener realizes audio reproduction that gives realistic sensation to the listener. However, it is difficult, in terms of space in many cases, to place multi-channel speakers of 5.1ch or 7.1ch at home. In view of the above, 3D acoustic technology that realizes quasi-acoustic effects identical to those produced by multi-channel audio reproduction using conventional stereo speakers has been developed.
For example, Patent Literature (PTL) 1 discloses a sound image localization apparatus that localizes a sound image at an arbitrary position by three-dimensional sound field processing. Moreover, PTL 2 discloses an acoustic signal reproducing apparatus that widens a sound image of a signal and thus reproduces the signal.